Crude oil devaporizing process



May 23, 1967 D. c. MEYERS CRUDE OIL DEVAPORIZING PROCESS v 2 Sheets-Sheet 1 Filed Nov. 25, 1964 VAPOR PRESSURE VS. TEMPERATURE (PARAFFIN HYDROCARBONS) IBIO WmLoFL :r L. .r 5mm 1 mwmwwmmm mom TEMPERATURE F lNVENTOR s H Y H E N M R a m T S A m m G U H O D W B n F May 23, 1967 Filed Nov. 25, 1964 :00- 33: so PSIG (consmr PRESSURE) 70- D. c. MEYERs 3,321,397

CRUDE OIL DEVAPORIZING PROCESS 2 Sheets-Sheet 2 VAPOR PRESSURES VS. TEMPERATURE (CRUDE OIL HYDROCARBON MIXTURES) 70 a0 9'0 I60 lio |io :50 do {so lo no TEMPERATURE "F FIG. 2

STORAGE TANK INVENTOR DOUGLAS C. MEYERS 9 8Y' ma Hi8 ATTORNEY United States Patent 3,321,397 CRUDE OIL DEVAPORIZING PRQCESS Douglas C. Meyers, Connersville, Ind, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Nov. 25, 1954, Ser. No. 413,787 2 Ciaims. (Cl. 208-351) This invention relates to a process for the devaporization of crude oil. More particularly, it relates to an improved process for recovering valuable crude oil vapors heretofore lost or recovered only through costly repressurizing systems attached to storage tanks.

Crude oil as produced from its reservoir consists of a mixture of many hydrocarbon components. The physical state of this crude oil, i.e., whether it is gaseous, liquid or in mixture thereof, depends primarily on the pressure and temperature to which the fluid is subjected. As the pressure is reduced in the movement of the crude oil from the reservoir to the storage (lease) tanks, gas and vapors are released as the result of the vapor-liquid phase equilibrium being maintained. Usually such gases and vapors are recovered from the remaining crude oil liquid, first, in oil and gas separators normally operated at pressures above atmospheric. Additional vapors produced as a result of a decrease in pressure on the crude fluid during storage, are usually at least partially recovered by vapor gathering lines and mechanical compressors, attached to each storage tank, which inject the recovered vapors into pressurized gas gathering systems.

The substantially complete recovery of these vapors, rich in liquifiable hydrocarbons such as propane, butane, natural gasoline etc., has become especially important in the recent years. Although the volume involved, e.g., about 20 to 50 cubic feet per barrel of crude oil produced, is relatively small, its recovery is of substantial importance and quite profitable, especially when large volumes of oil are produced.

Elaborate schemes have been employed to recover these valuable light hydrocarbons. For example, in US. 2,773,- 559, crude oil is first treated in a stabilizer at reduced pressure, followed by further pressure reduction and heating wherein a portion of the vapor is removed through a separator with compression and recycle of the removed vapor to the first stabilizer. Such conventional methods of recovering tank vapors only justify the cost of installation and operation when a large amount of crude oil is being stored. Therefore, because of the additional expense in small oil leases, no attempt has been made to recover these hydrocarbon vapors which are, of course, then lost.

It is therefore the primary object of this invention, to recover crude oil components which are gaseous at atmospheric pressure and ambient temperature. It is a fur ther object of this invention to recover these vapors prior to storing the crude oil in lease tanks. Other objects will be apparent and the objects better understood from the description of the invention which follows and by refere ce to the accompanying drawings, wherein:

FIGURE 1 is a graph showing typical temperaturepressure curves for random ambient temperature/atmospheric pressure hydrocarbon compositions;

FIGURE 2 is a graph showing typical vapor pressure/ temperature curves for various crude oil compositions at ambient temperature; and

FIGURE 3 is a simplified flow diagram illustrating a preferred mode of practicing this invention.

Now, in accordance with the present invention, it has been found that gaseous hydrocarbons can be substantially completely recovered by retaining the crude oil at the elevated pressure under which it is released from the 3,3'Zi,37 Patented May 23, 1967 oil Well, while increasing the temperature to such a level that its vapor-liquid equilibrium will be maintained under such conditions that this equilibrium will be represented by the same curve that such a liquid composition is represented by when at atmospheric pressure and ambient temperature, then cooling the remaining devaporized crude oil to ambient temperature and reducing the pressure to atmospheric on transfer to storage tanks, thereby recovering all the valuable liquifiable hydrocarbons heretofore lost in storage operations.

By establishing equilibrium at an elevated pressure and temperature, an under-saturated condition results at the eventually reduced pressure and temperature, such as atmospheric pressure and ambient temperature, so that there is no flashing of vapors when the devaporized crude oil is transferred from the separators to the lease (storage) tanks. Since vaporization is a function not only of pressure but also of temperature, vapors have usually been released in the storage tanks due to the reduction of elevated pressure maintained in conventional oil and gas separators to that of the storage tanks (atmospheric pressure). In the process of this invention, these desirable vapors are driven off at the separator pressure and at some elevated temperature and thus are recovered in the separator gas gathering system. The release of additional vapor upon reduction of pressure in the storage tanks is prevented by cooling the crude oil to ambient (storage tank) temperature prior to pressure reduction, or conversely, by removing all the vapor which would be released at ambient temperature and atmospheric pressure prior to subjecting the petroleum fluid to these conditions of temperature and pressure.

Temperature data for paraffin hydrocarbons, as illustrated by the standard curves illustrated by FIGURE 1, show that any crude oil containing mixtures of the various hydrocarbons will react similarly to that of the pure hydrocarbon components as far as vapor pressure/temperature relationship is concerned. For example, pure pentane at atmospheric pressure has a vaporization temperature of 97 F. while at an elevated pressure, for example, e.g., p.s.i.a., the vaporization temperature would be 164 F. Crude oil, containing a mixture of practically all hydrocarbons, will be in equilibrium at any ambient temperature and atmospheric pressure, with the composition at equilibrium and atmospheric pressure being a function of ambient temperature. Crude oil thus has multitudes of vapor pressure-temperature curves normally falling between those for pure butane and pure pentane, all of which are approximately parallel to the curves for the two pure components.

Referring to FIGURE 1, the graph shows temperature versus vapor pressure curves for typical hydrocarbon components. As shown on this graph, a crude oil in equilibrium at an atmospheric pressure and ambient temperature of 76 F. (point B on the graph) would not produce additional vapors or change in composition at p.s.i.a. until the temperature was elevated to 144 F. (point C on the graph). Crude oil in equilibrium at a separator pressure of 55 p.s.i.a. and an ambient temperature of F. (point A on the graph) changes to equilibrium at atmospheric pressure and ambient temperature (point B on the graph) when oil is released from, for example, the separator to the tank, resulting in evaporization of some of the crude oil components in order to produce equilibrium at the lower pressure, thus producing tank vapors. As illustrated by the graph, the crude oil compositions existing at equilibrium point B, will exist at point C if the oil, at separator pressure-ambient temperature equilibrium (point A) is heated at constant pressure to 144 F. (point C). This causes evaporization of the identical components that would be released by the crude oil at the separator conditions of point A when reduced in pressure from point A to point B since the 3 vapor-equilibrium composition at point B and point C are identical. Therefore, vapors normally released when a crude oil composition is transferred from the conditions of temperature and pressure illustrated by point A to the conditions of temperature and pressure illustrated at point B, will be recovered in the separators at separator pressure rather than at atmospheric pressure in a complex system attached to the storage tanks. Therefore, the instant invention is based on the discovery that it is more economical to prevent vapors from reaching the storage tanks than to recover the vapors from the storage tanks. Again referring to FIGURE 1, a crude oil under conditions of temperature and pressure as illustrated at point C can either be cooled to ambient temperature (70 F.) and reduced to atmospheric pressure either by simultaneously reducing the temperature and pressure as illustrated by the curve between the points C and B or alternately cooled as illustrated by the line from point C to point A at separator pressure and then reduced in pressure as is illustrated by the line from point A to point B at ambient temperature, so as not to produce additional vapors.

In retrospect it is clear that for any crude oil, the desired vapor pressure-temperature curve will be determined by the ambient temperature. Furthermore, the temperature to which the oil must be heated becomes a function of the ambient temperature and separator pressure. For example, an ambient temperature of 70 F. will give a curve corresponding to curve C contained in the graph included on FIGURE 2. If, for example, the separator pressure is 60 p.s.i.a. or approximately 45 p.s.i.g., the oil having a temperature-pressure curve corresponding to curve C in FIGURE 2 must be heated to 150 F. to drive off vapors which would be released when the pressure is subsequently reduced from 45 p.s.i.g. to atmospheric.

Since atmospheric and separator pressures will remain fairly constant in a specific geographic area where a particular lease is present and since curves such as illustrated in FIGURE 2 show a direct linear relationship between variation in ambient and required elevated temperature, it is possible to control the desired set point of a temperature controller as a function of ambient temperature. Thus, a differential temperature controller could automatically regulate the temperature to which a crude oil would have to be heated as the ambient temperature varied. For example, with a 45 p.s.i.g. separator pressure, differential of 80 F. is required regardless of the ambient temperature. For a separator pressure of 60 p.s.i.g. a differential of 94 F. is required while at 30 p.s.i.g. a differential of only 60 F. is required. I

Referring to FIG. 3, petroleum fluid from well 1 is passed via line 3 to a heat exchanger 5 wherein the temperature of the crude oil is raised so that it is at least sufficient to produce the vapor-liquid equilibrium which would be present, were the petroleum fluid at atmospheric pressure and ambient temperature. The heated petroleum fluid is then passed via line 7 to a separator 9 wherein gas and vapors are removed via line 11 for further treatment. Any water contained with the petroleum fluid is separated and removed via line 13. The heated petroleum fluid, still under wellhead pressure, is removed via line 15 and passed to a heat exchanger 17 wherein the temperature is reduced, preferably to ambient temperature. Thereafter, the cooled petroleum fluid is passed via line 19 to storage tank 21, preferably maintained at ambient temperature and atmospheric pressure.

Details as to relative sizes, shapes and placement of the pieces of equipment and provision for gas compressors, valves, baffles, fluid seals, condensors, heaters and the like are omitted for clarity since they will be readily supplied by those skilled in the relevant art.

By way of illustration, two oil wells, one producing 50 barrels of crude petroleum oil per day and 50 barrels of salt water per day at 40 lbs. per square inch guage pressure and 130 F. and the other producing 50 barrels of crude petroleum oil per day at 40 lbs. per square inch guage pressure and F. are together passed to a heat exchanger wherein the temperature of the combined petroleum crude is raised to 140 F. This heated mixture is treated in a separator wherein gas and vapors are removed overhead, the salt water is removed and passed as make-up coolant to a salt water pond having a temperature of 75 F. The liquid petroleum fluid in the separator is passed into a conventional heat exchanger containing 400 feet of Z-inch pipe included in the salt water pit. In this manner, barrels per day of crude oil can be cooled to ambient temperature.

The following specific examples of the invention will serve to illustrate more clearly the application of the invention, but are not to be construed as in any manner limiting of the invention.

Example I.Relative volume of vapors lost by conventional oil handling system Samples of oil were obtained under pressure from an oil field wherein the release separator was operating at 220 p.s.i. and 76 F.

Three liters of the sample were transferred from a sample bomb to a test bomb, pressurized to 200 p.s.i. In order to simulate actual field conditions the crude oil was slowly dumped from the test bomb into a five-gallon tank jug. Constant pressure was maintained on the test bomb by means of water admitted into the bottom of the bomb. The five-gallon tank jug was held at zero p.s.i.g. with the vapors being measured by means of a water displacement jug.

Over a period of approximately four hours it was found that 18,800 ml. of water was displaced from measurement jar when all the oil in the test bomb had been dumped into the tank jar or a net vapor volume of 18,8003000=15,800 ml. This gave a vol/vol. ratio of 5.3:1 or 30.8 cu. ft. per bbl.

Example II. -Prevention of vapor loss by heating under pressure A three liter sample of lease crude obtained at the same time and under the sample conditions as in Example I was introduced into the test bomb at 200 p.s.i.g. The pressure on the test bomb was lowered to 30 p.s.i.g. (the pressure of a lease heater treater) by withdrawing water from the test bomb to hold pressure constant for a period of an hour and a half. During this period a total of 2660 ml. of water was recovered from the bomb for an equivalent vapor volume of 8350 ml. of at zero p.s.i.g.

The heater in the test bomb was then turned on and the temperature of the oil raised from 70 F. to F. in a period of one hour. The pressure on the oil was held constant by bleeding off the vapors from the test bomb into the tank jug and vapors measured by water displacement in the meter jug. A maximum of 3900 ml. of vapors were measured when crude sample was heated to 112 F. but vapors condensed as fast as they were released from the test bomb above this temperature and the accumulated total volume of water displaced from jug was only 3850 ml. when sample was heated to 140 F. or 50 ml. less than at 112 F. Thus a total vapor volume of at least 12,200 ml. could be recovered under pressure in form of a gas.

The vapors in the test bomb were bled off and heater turned off and sample cooled back down to 80 F. wtih pressure held constant at 30 p.s.i.g. by admitting water to test bomb. At 80 F. the oil sample was dumped into the tank jug and vapors resulting from dropping pressure to zero p.s.i.g. were measured. Only 1210 ml. of water was displaced from metering jug which was actually less than the 3000 ml. volume of sample added to the tank jug or a net reduction in vapors. Since the tank jug still contained saturated rich vapors from previous test and since oil after being cooled may have been undersatua'ated, it is believed the oil may have actually absorbed some of the vapors originally contained in the tank jug.

Thus, for equilibrium condition, there was no apparent loss in vapors from the simulated processing system except for the 1210 ml. due to gas displacement by the oil volume.

I claim as my invention:

1. A process for separating the vaporous components contained in a multiphase petroleum fluid obtained from a high pressure Well, comprising:

(a) subjecting said petroleum fluid to a temperature to produce the vapor-liquid equilibrium which would be present, were said petroleum fluid at atmospheric pressure and ambient temperature while retaining it at the wellhead pressure, thereby releasing volatile hydrocarbons;

(b) separating said volatile hydrocarbons from the remaining devaporized petroleum fluid while maintaining said remaining devaporized petroleum fluid at the Wellhead pressure;

(c) cooling said remaining devaporized petroleum fluid to ambient temperature while maintaining said remaining devaporized petroleum fluid at the Wellhead pressure; and

(d) thereafter venting said remaining devaporized petroleum fluid to an atmospheric pressure storage facility.

2. A process for separating the vaporous components contained in a multiphase petroleum fluid obtained from a high pressure well, comprising:

(a) subjecting said petroleum fluid to a temperature to produce the vapor-liquid equilibrium which would be present, were said petroleum fluid at atmospheric pressure and ambient temperature while retaining it at the wellhead pressure, thereby releasing volatile hydrocarbons;

(b) separating said volatile hydrocarbons from the remaining devaporized petroleum fluid while maintaining said remaining devaporized petroleum fluid at the wellhead pressure; and

(0) simultaneously cooling and venting said remaining devaporized petroleum fluid to ambient temperature and to an atmospheric pressure storage facility.

References Cited by the Examiner UNITED STATES PATENTS 2,225,949 12/1940 Bennett 208351 2.773,559 12/1956 Cottle 208351 DELBERT E. GANTZ, Primary Examiner.

25 H. LEVINE, S. P. JONES, Assistant Examiners. 

1. A PROCESS FOR SEPARATING THE VAROROUS COMPONENTS CONTAINED IN A MULTIPHASE PETROLEUM FLUID OBTAINED FROM A HIGH PRESSURE WELL, COMPRISING: (A) SUBJECTING SAID PETROLEUM FLUID TO A TEMEPRATURE TO PRODUCE THE VAPOR-LIQUID EQUILIBRIUM WHICH WOULD BE PRESENT, WERE SAID PETROLEUM FLUID AT ATMOSPHERIC PRESSURE AND AMBIENT TEMPERATURE WHILE RETAINING IT AT THE WELLHEAD PRESSURE, THEREBY RELEASING VIOLATILE HYDROCARBONS; (B) SEPARATING SAID VOLATILE HYDROCARBONS FROM THE REMAINING DEVAPORIZED PETROLEUM FLUID WHILE MAINTAINING SAID REMAINING DEVAPORIZED PETROLEUM FLUID AT THE WELLHEAD PRESSURE; (C) COOLING SAID REMAINING DEVAPORIZED PETROLEUM FLUID TO AMBIENT TERMPERATURE WHILE MAINTAINING SAID REMAINING DEVAPORIZED PETROLEUM FLUID AT THE WELLHEAD PRESURE; AND (D) THEREAFTER VENTING SAID REMAINING DEVAPORIZED PETROLEUM FLUID TO AN ATMOSPHERIC PRESSURE STORAGE FACILITY. 